1. Field of the Invention
This invention relates to a method and apparatus for controlling sample temperatures, and more particularly to a method and apparatus for controlling the temperature of a sample being processed in a vacuum at a predetermined temperature.
2. Description of the Prior Art
It is one of the important uses of apparatuses such as a dry etching apparatus for processing samples in a vacuum, for instance, utilizing plasma (hereinafter called plasma process) to form minute patterns in the manufacture of minute solid elements, including semiconductor integrated circuits and the like. The formation of such minute patterns is normally carried out by exposing a high polymer material known as photoresist, applied to a semiconductor substrate (hereinafter called substrate) as a sample, to ultraviolet rays and transferring the pattern as a mask obtained by developing it to the substrate by means of dry etching.
The mask and substrate are heated by the collision energy of ions or electrons in the plasma and by the chemical reaction energy during the dry etching for the substrate. Accordingly, unless the substrate temperature is favorably controlled at the time of etching the mask will deform and degenerate because sufficient heat radiation is not possible, thus making it unavailable to be a proper pattern and making it difficult to remove the remaining mask from the substrate after dry etching. To eliminate these disadvantages, techniques for controlling the substrate temperature have been generally employed and proposed. The conventional techniques will be described as follows:
As one of the examples on the prior art, there has been published Japanese Pat. Gazette No. 56-23853. This technique comprises water cooling of a sample stand to which the output of a high frequency power supply is applied, mounting a substrate as a sample on the sample stand through an insulating material, giving a potential difference to the insulating material through plasma by applying d.c. voltage to the electrode, causing the substrate to adhere to the sample stand by means of the electrostatic attraction thus generated and effectively cooling the substrate by reducing heat resistance across the substrate and the sample stand. However, the contact area between the substrate and the insulating material is small even in this technique and there is a small gap therebetween from a microscopic view. Moreover, the processing gas is allowed to enter the gap and it will be converted into heat resistance. In the dry etching apparatus in general, a substrate is normally subjected to dry etching under the processing gas pressure at about 0.1 Torr and, since the gap between the substrate and the insulating material becomes smaller than the mean free path length, reduction in the gap by the electrostatic attraction rarely changes from the heat resistant point; in other words, it will become more effective to the extent that the contact area has been increased. Accordingly, greater electrostatic attraction is required to reduce the heat resistance across the substrate and the sample stand and to cool the substrate more effectively. For this reason, with such techniques, the following problems must be considered.
(1) The time required for conveying the substrate that has been subjected to the etching process tends to increase and the substrate may be damaged because the substrate is difficult to remove from the sample stand.
(2) Although a greater potential difference must be given to the substrate to cause a greater electrostatic attraction to be generated, the greater potential difference allows elements in the substrate to be further damaged and consequently sufficient throughput is unavailable in the delicate process of creating of ever thinner gate films in view of element production.
As another example of the prior art, there has been published Japanese Pat. Gazette No. 57-145321. According to this technique, the substrate is directly cooled by a gas. It is, needless to say, possible to improve cooling efficiency with a cooling gas such as helium (hereinafter called GHe) offering excellent thermal conductivity. However, there are the following disadvantages inherent in that technique:
(1) Even such an inert gas as GHe seriously affects the process because a large amount of cooling gas flows not only on the cooling face of the substrate but also in the process chamber; consequently, it may not be used for the entire process.
As still another example, there has been proposed a technique disclosed by E. J. Egerton et al in Solid State Technology, Vol. 25, No. 8 pp 84.about.87 (August, 1982). In this technique, GHe at a pressure of 6 Torr is caused to flow between a water cooled sample stand as an electrode and a substrate mounted on and fixed to the electrode by pressing the substrate on the periphery thereof using mechanical clamp means to reduce heat resistance between the electrode and the substrate and effectively cool the substrate.
However, even according to such a technique, the inflow of the GHe to the process chamber will be unavoidable and there are problems similar to those enumerated in the second example and there are the following disadvantages.
(1) Since only the periphery of the substrate is pressed against and fixed to the electrode, the substrate will deform in a convex shape with the periphery being firmly fixed under the pressure of GHe. Consequently, the gap between the under surface of the substrate and the electrode is increased and thermal conductivity across the substrate and the electrode will be reduced; consequently, the substrate is not effectively sufficiently cooled.
(2) Since there is provided mechanical clamp means for pressing and fixing the substrate against and to the electrode, the effective area for preparing elements in the substrate will be reduced and the conveyance of the substrate will become complicated, which will result in increased apparatus size and decreased reliability.
(3) Uniformity of plasma is impaired by the mechanical clamp means.
(4) When the mechanical clamp means operates, products generated by the reaction may fall off the clamp and generate dust, causing serious trouble in processing the substrate.
(5) An uneven distribution of GHe will occur in the gap between the under surface of the substrate and the electrode.